Disorders of perception, attention, and memory frequently accompany the major mental diseases. To begin to understand the neural mechanisms of these mental processes, we are recording the activity of neurons in the extrastriate cortex of monkeys engaged in tasks requiring visual discrimination, selective attention, and recognition memory. We found previously that selective attention gates visual processing by filtering unwanted information from the receptive fields of extrastriate neurons. Recently, we found that even the degree to which attended stimuli are processed by extrastriate neurons depends on "how much" attention or effort is devoted to them. When an animal is challenged by a difficult task, it appears to "rise to the occasion" by concentrating its attention, a correlate of which is sharpened selectivity of the neurons processing incoming sensory information. To identify the mechanisms by which cognitive state modulates cortical activity, we are examining both extrastriate neuronal activity and animal behavior in an attention-demanding task following lesions or chemical deactivation of structures that provide inputs to extrastriate cortex. These studies have identified two structures, each of which may play a different role in attentional modulation of cortical activity. One, the lateral pulvinar, appears to be critical for the ability to focus attention on a single stimulus and to ignore distracting stimuli. The other, the posterior parietal cortex, plays a more important role in spatial perception and the ability to shift attention from one location to another.